Electrode material for hydrocarbon sensors

ABSTRACT

The invention relates to an electrode material for potentiometric or amperometric electrochemical sensors with the chemical composition Ln 1-2  A 1-x  B x  O 3 , where Ln is at least a lanthanide cation or a mixture of rare earth cations, A is at least a trivalent transitional material, and B is at least a trivalent or bivalent redox-stable cation.

The invention relates to a novel electrode material for hydrocarbonsensors, as well as a novel sensor and a procedure for its manufacture.

As is known, the concentration of nonburned fuels in oxygenous gases canbe determined directly by means of sensors in the combustion gas flowwhich feature two electrodes on a solid electrolyte, e.g.,yttrium-stabilized zirconium dioxide, which respond in different ways tothe test gas. As the potential of one of the electrodes is largelydetermined by the equilibrium oxygen partial pressure of the gas, avoltage level between the electrodes in the same gas can be determinedwhich is dependent on the hydrocarbon concentration. Preferably, goldand alloys of gold and platinum are used as CHx-sensitive electrodes(c.g., A. Vogel, G. Baier, V. Schule, Sensors and Actuators 15-16 (1993)147-150).

A disadvantage of these types of arrangements is that gold electrodesare morphologically unstable over time at the relatively high operatingtemperatures of the cells (≧700° C.) and that, consequently, thedeveloping potential is subject to changers over time. Anotherdisadvantage is that a potential jump generally takes place with thesetypes of electrodes when the stoichiometric ratio (of λ=1) is exceeded.In addition, the potential of these types of electrodes is dependentupon pre-treatment with respect to gas admission and temperature, sothat memory effects occur, which must be eliminated by constantcalibration during the use of electrodes in sensors.

As is known, the electrode material may consist of mixed oxides of theperovskite type, which are generally well-known and well-researched asan oxygen electrode, and are used as materials for electrodes which,preferably, only convert oxygen electrochemically. These types ofelectrodes are not known as burnable gas sensitive electrodes.

The object of the invention is to provide an electrode material for asensor which is stable over long periods of time, as well as a sensorwhich is stable over long periods of time.

This object is solved by an electrode material of the invention.

Because a material with the chemical composition Ln_(1-z) A_(1-x) B_(x)O₃ is provided as the electrode material for potentiometric oramperometric, electrochemical sensors, where Ln is at least lanthanidecation or a mixture of rare earth cations, A is at least a trivalenttransitional material, and B is at least a trivalent or bivalentredox-stable cation, an electrode with a perovskite structure can beprovided for an electrochemical sensor which is stable over long periodsof time, even is aggressive high-temperature environments, once it hasbeen sinter-fused onto a solid electrolyte or a ceramic base material.

In this regard, particularly favorable burnable gas sensitivity isachieved (with burnable gas being generally defined as a component whichis gaseous or oxidizable under the operating conditions of the sensor)if the element or the element mixture A and/or the element or theelement mixture B exhibits low catalytic activity.

In this regard, it is particularly advantageous if A consists of anelement or a mixture of elements from the group comprising manganese,chromium, cobalt, iron and titanium. Preferably, the element or theelement mixture B is selected from the group comprising gallium,aluminum, magnesium, calcium, gadolinium, and other redox-stable rareearth elements.

In particular, it is advantageous if A is manganese or chromium or amixture of the two and if B is an element or a mixture from the groupcomprising gallium, aluminum, and magnesium. An electrode material inwhich A is chromium and B is gallium has proven to be particularlyadvantageous.

Furthermore, it is advantageous if Ln is a lanthanide or a mixture oflanthanides, particularly lanthanum itself, which exhibits especiallyadvantageous properties is relation to this invention.

The parameter x ranges from 0.001 to 0.99, particularly 0.1 to 0.9,preferable 0.1 to 0.8 or 0.2 to 0.5, with a range of 0.15 to 0.25 and,particularly, 0.19 to 0.21, having proven to be particularlyadvantageous.

The parameter z is either equal to 0 or ranges from 0.01 to 0.29, from0.3 to 0.6, or from 0.19 to 0.4.

If desired, the hydrocarbon sensitivity of the electrode can beincreased by selecting the parameter z in relation to compounds wherez=0. Due to intentional generation of under-stoichiometry of the Lnions, oxide ion vacancies are formed in the oxygen partial matrix of thecompound. As a result, sensitivity is based on other electrodemechanisms than those appearing in compounds where z=0.

The sensitivity of the electrode is additionally increased within arange of z=0.3 to z=0.6. As z increases, the element A may be present inan oxidic phase in addition to the mixed oxide, so that the electrode asa whole exists in a mixed phase.

The element or the element mixture B can also exist heterogeneously asan oxide in addition to the mixed oxide, comprising between 0.1% and 70%of the whole.

A sensor for burnable gases based on the invention, particularly forhydrocarbons, features a solid electrolyte and at least 2 electrodes,with at least one electrode containing an electrode material with theadvantageous features described above.

According to one embodiment of the invention, the second electrode hasthe same chemical composition as the first electrode. In this case,favorable burnable gas sensitivity is achieved by providing means forgenerating temperature difference between the first and the secondelectrode. Advantageously, the temperature difference during operationbetween the first and the second electrode should range from 100° C. to200° C.

In sensors in which the chemical composition of the electrodes is notidentical, the chemical composition of the second electrode is,advantageously, Ln_(1-y) C_(y) DO₃, where Ln is an defined above, C isan alkaline earth metal, and D is at least a trivalent transition metal.In particular, C is strontium. Advantageously, D is manganese and/orchromium. Ln may be a different element or a different mixture ofelements in the second electrode than it is in the first electrode.

Advantageously, the parameter y ranges from 0.01 to 0.9 and,particularly, from 0.02 to 0.7, 0.05 to 0.5, 0.1 to 0.3 or 0.2 to 0.4.

A process for the manufacture of a sensor for burnable gases whichemploys an electrode material according to the invention comprises thefollowing steps:

Bonding of the initial products containing Ln, A and B, preferable asLn₂ O₃, A₂ O₃ and B₂ O₃, possibly with solvents;

Conversion of the mixture at about 1,350° C. to 1,650° C. to form aconversion product;

Pulverization of the conversion products;

Production of a paste; and

Imprinting and burning the paste onto a base material.

Instead of the oxide, the initial material selected may, for example,consist of the applicable citrate or nitrate compound of the initialsubstances.

The base material may be a solid electrolyte. However, printing may beperformed directly onto an oxide ceramic base material, such as Al₂ O₃,and a solid electrolyte may be placed over or adjacent to the printedmaterial. H₂ O and/or other organic solvents are advantageously used assolvents. Hydrophilic or hydrophobic organic solvents may be used. Safeand complete oxidation of the electrode material is guaranteed onceconversion to air or oxygen has occurred. If the conversion productforms a sinter product, another annealing step may take place followingpulverization of the conversation product. This ensures completehomogeneous reaction of the components. In industrial manufacturing, thecomponent Ln₂ O₃ may also be a mineral, such as cerite earth. Acomposition largely corresponding to monazite is particularlyadvantageous.

A sensor according to the invention or a sensor which has beenmanufactured according to the advantageous process may find application,in particular, as a hydrocarbon sensor for use in exhaust gas or in acombustion unit, where the combustion unit may be an internal combustionengine with internal or external combustion (particularly a sparkignition engine or a diesel engine) or a heating system, such as an oilor natural gas heating system.

In this regard, favorable hydrocarbon sensitivity may be achieved if thesolid electrolyte is made of a fully stabilized ZrO₂ containingapproximately 8 mol.- % Y₂ O₃. As a result of dotation with Y₂ O₃,vacancies are formed in the oxygen partial matrix of the solidelectrolyte.

However, it is also possible to affix the electrodes to a solidelectrolyte with a low degree of Y₂ O₃ dotation or the correspondingproportion of another cation with low valence, e.g., Mg or Ca, i.e., onehaving a low concentration of vacancies.

The vacancy concentration of the initial solid electrolyte can also bereduced by means of chemical modification with equivalent or superiorcations. The addition TiO₂ or Nb₂ O₅ is advantageous. The use of acompletely different type of solid electrolyte, conceivably NASCION, isalso an option.

A detailed description of the invention follows which is base on thepreparation of three exemplary electrode materials according to theinvention.

EXAMPLE 1

To produce the compound LaCr₀.80 Ga₀.20 O₃, the oxides La₂ O₃ H₂ O, Cr₂O₃, and Ga₂ O₃ weighed in terms of their stoichiometric ratios and aremixed in a ball type mill for 20 minutes. The mixture is then convertedin air in a sintered corundum converter at 1400° C. for 20 hours. Theresulting sinter cake is mortared and annealed at 1650° C. for 30minutes. Complete formation of the desired product can be demonstratedby means of X-ray diffractometer images.

EXAMPLE 2

To produce the compound LaCr₀.80 Al₀.20 O₃, the oxides La₂ O₃ H₂ O, Cr₂O₃, and Al₂ O₃ in terms of their stoichiometric ratios, mixed in a balltype mill for 20 minutes, and then converted in air in a sinteredcorundum converter at 1400° C. for 20 hours. The resulting sinter cakeis mortared and subjected to an annealing process at 1650° C. for 30minutes to obtain the purest possible compound.

EXAMPLE 3

To produce an oxygen-sensitive perovskite electrode, the compoundLa₀.995 Sr₀.005 CrO₃ is weighed in terms of the stoichiometric ratios ofthe oxides La₂ O₃ H₂ O, Cr₂ O₃, and SrCO₃ and mixed in a ball type millfor 20 minutes. The mixture is then converted in air in a sinteredcorundum converter at 1400° C. for 20 hours. The resulting sinter cakeis mortared and annealed at 1650° C. for 30 minutes.

Using a basic, electronics-compatible layering technique, such as screenprinting, the electrode materials based on example 1 and example 2 canbe imprinted onto a solid electrolyte and can then be used next to aso-called equilibrium electrode, such as one made of platinum.

The electrode material based on example 3 forms a mixed oxide of theperovskite type with negligible burnable gas sensitivity. It can be usedin place of the platinum electrode used next to the electrodes describedin example 1 and example 2. The sensors, which, according to examples 1and 2, can be manufactured as burnable gas-sensitive electrodes, or,according to example 3, as equilibrium electrodes, show no signs of thevoltage jump which occurs at λ=1 with platinum electrodes, acharacteristic feature of conventional λ sensors. Thus, we obtain aburnable gas sensitive sensor which is suitable for consistent use inthe vicinity of λ=1 in the exhaust gas of an automobile engine featuringa regulated catalytic converter. The signal emitted by the sensoraccording to the invention is primarily dependent of the concentrationof burnable gases in this exhaust gas, i.e., on hydrocarbons which havenot been fully burned or after burned by the catalytic converter. Theconstant transition from λ>1 to λ<1 and vice-versa, which is related tothe regulating processes of the engine control system, does not obstructor only slightly obstructs the output signal of the sensors according tothe invention.

It is also possible to manufacture burnable gas sensors using twocompletely identical burnable gas sensitive electrodes, such as LaCr₀.8Ga₀.2 O₃, by operating the two electrodes at a temperature at whichburnable gas sensitivity disappears (and at which the electrode exhibitssufficient catalytic activity). This electrode then becomes an oxygenelectrode. The second electrode is operated at a temperature at whichthe electrode material is not yet catalytically active. Consequently,burnable gas sensitivity is retained with this type of electrode. Tothis end, a sensor can be manufactured in which the two electrodes areaffixed to the same substrate, but in which a temperature drop is setacross the sensor, thereby producing a temperature difference betweenthe electrodes of 100 K to 150 K. This temperature drop may, inparticular, be generated by a heat conductor printed onto the basematerial of the sensor. The advantage of this embodiment consists in thefact that both electrodes can be imprinted onto the substrate or ontothe solid electrolyte in a single step of the manufacturing process.

It is also possible to apply voltage to the electrochemical cellscontaining the burnable gas sensitive electrodes based on example 1 orexample 2, on the one hand, and the oxygen electrodes based on example 3or a platinum electrode, thereby forcing a flow of current which isdirectly related to the concentration of burnable gas. This mode ofoperating the sensors according to the invention is known in the art andis referred to as an amperometric mode of operation.

In connection with the above description, it may be assumed thatlanthanides are elements with ordinal number 57 to 71, trivalenttransition metals are elements with ordinal number 21 to 28, 39, 41, 42,44, 45; 57 to 71, 74, 76, 77, 79, and 92, and, finally, redox-stablecations are such elements as Ga, Al, Sc, Mg and Ca.

What is claimed is:
 1. An electrode material for potentiometric oramperometric electrochemical sensors with the chemical compositionLn_(1-z) A_(1-x) B_(x) O₃, where Ln is at least a lanthanide cation or amixture of rare earth cations, A is an element or a mixture of elementsselected from the group consisting of Mn, Cr, and Co; B is an element ora mixture of elements selected from the group consisting of Ga, Al, andGd; x has a value of 0.001 to 0.99; and z has a value of 0 to 0.6.
 2. Anelectrode material of claim 1 wherein A is chromium.
 3. An electrodematerial according to claim 1 wherein B is gallium.
 4. Electrodematerial according to claim 1, wherein Ln is an element or a mixture ofelements from the group consisting of La, Nd, Sm, Eu, Gd, Dy, Ho, Er,Tm, Yb, and Lu.
 5. Electrode material according to claim 1, wherein Lnis an element or a mixture of elements from the group consisting of rareearth elements.
 6. Electrode material according to claim 1, wherein Lnis La.
 7. Electrode material according to claim 1, wherein x fallswithin a range between 0.1 and 0.9.
 8. Electrode material according toclaim 1 wherein x falls within range between 0.1 and 0.8.
 9. Electrodematerial according to claim 1, wherein x falls within a range between0.2 and 0.5.
 10. Electrode material according to claim 1, wherein xfalls within a range between 0.15 and 0.25.
 11. Electrode materialaccording to claim 1 wherein x falls within range between 0.19 and 0.21.12. An electrode material according to claim 1 wherein oxides of B arepresent in addition to the chemical composition Ln_(1-z) A_(1-X) B_(X)O₃, within a range between 0.01 wt. % and 70 wt. % of the electrodematerial.
 13. Electrode material according to claim 1, wherein z fallswith in a range between 0.01 and 0.29.
 14. Electrode material accordingto claim 1, wherein z falls within a range between 0.3 and 0.6. 15.Electrode material according to claim 1, wherein z falls within a rangebetween 0.19 and 0.4.
 16. Sensor for burnable hydrocarbon gases,comprising a solid electrolyte and at least two electrodes, wherein atleast one electrode comprises an electrode material according toclaim
 1. 17. Sensor according to claim 16, wherein the second electrodehas the same chemical composition as the one electrode.
 18. Sensoraccording to one of the preceding claim 16, wherein means are providedfor generation of a temperature difference between the electrodes. 19.Sensor according to claim 18, wherein the temperature difference duringoperation between the electrodes is 100 K to 200 K.
 20. A sensor forhydrocarbon in the exhaust gas of a combustion unit, said sensor havinga solid electrolyte and at least two electrodes, in which at least afirst electrode of the two electrodes consists essentially of anelectrode material in accordance with claim 1.